This invention relates to protective breathing apparatus of the type in which a user wears a face mask, sometimes referred to as a respiratory inlet covering, communicating with a source of air or other breathing fluid for use in toxic or oxygen deficient surroundings. More specifically, this invention is directed to such apparatus of the pressure demand type, in which the breathing fluid is provided on demand, and is maintained within the mask at a positive pressure, that is a pressure above atmospheric whereby any leakage caused by poor fit or component failure will be outwardly from the mask, to prevent inflow and possible inhalation of a toxic ambient atmosphere.
However, a problem arises whenever the mask of such pressure demand apparatus is not in place on the face of the wearer, unless the air supply has been manually shut-off. This is because the face of the wearer is required to define the mask chamber within which the positive pressure is to be maintained. If the air supply is not manually shut off, and the mask is off the face and open to the atmosphere, the mask chamber becomes infinitely large and the apparatus cannot maintain a pressure above atmospheric pressure within that chamber. However, the apparatus seeks to do so and the air supply is quickly depleted.
It is known to provide pressure-demand systems which means for manually switching to a straight demand mode. If this is done, the user must remember to switch back to the pressure-demand mode for maximum protection.
The evolution of user and buyer requirements as well as those of various regulatory agencies has seen an upward spiral of flow requirements such that modern regulators, in fully open position, can discharge enormous quantities of air as compared to the normal breathing requirements of a man. Over 500 liters per minute (17.6 cfm) is not unusual as a free flow regulator performance although the minimum approved quantity is 200 liters per minute. During donning and doffing or inadvertant removal of the mask the high flow will occur unless the air supply is off. It is difficult to don or doff and simultaneously turn the air on or off, and if the mask is forced off the wearer's face, for example during a fall, he may not be in a condition to immediately refit the mask or manually shut off the air supply. It is therefore desirable to provide an automatic shut-off of the air supply in such situations where mask back pressure is lacking to prevent escape and rapid wasteful depletion of the limited air supply.
In pending United States patent application Ser. No. 926,004, filed July 19, 1978 in the name of John L. Sullivan, one of the inventors herein, there is disclosed a pressure demand breathing apparatus having an air shut off operable automatically under abnormal flow conditions which occur when the mask is off the face of the user, to interrupt the flow of air to the mask and thereby conserve the air supply. The shut off device is designed to remain open during air flow at rates up to a predetermined rate selected as the maximum flow rate expected to be encountered under normal conditions of use, and to close at flow rates exceeding that preselected rate which are produced when the mask is not in place.
Occasionally a particular user, operating under conditions of extreme stress, requires flow rates momentarily peaking above the preselected rate. It is desirable that the automatic shut off device accommodate such user induced excess flow rates to avoid interrupting the air supply to such a user.